Human dilated cardiomyopathy (DCM) is associated with increased Gi protein levels, increased Gi signaling, and auto-antibodies that activate signaling by GI- coupled receptors. The goal of this proposal is to test the hypothesis that Gi signaling can cause DCM. Control of Gi signaling has been achieved by expressing in the heart a Gi-coupled receptor that has been specifically designed to be a Receptor Activated Solely by a Synthetic Ligand, or RASSL. The first RASSL (R1) is based on a Gi-coupled, kappa- opioid receptor. R1 contains mutations that reduce affinity for natural peptide agonists and yet allow activation by the drug spiradoline. Cardiac-specific, conditional expression of R1 in transgenic mice is achieved with a tetracycline-controlled expression system utilizing the a-myosin heavy chain promoter. Activation of R1 signaling by spiradoline administration results in acute slowing of heart rate and complete atrioventricular block, which are known effects of Gi signaling. Preliminary studies show that prolonged signaling by R1 causes a lethal form of congestive heart failure with anasarca (up to 60% weight gain), contractile dysfunction, and the histopathological features of DCM. The Gi signaling-induced cardiomyopathy can be phenotypically reversed by suppressing R1 expression, creating the potential for studies of disease recovery as well as disease onset. Specific Aims are: (1) to determine if receptor- stimulated Gi signaling in the heart can cause the characteristic anatomical, physiological and histopathological changes of DCM using echocardiography, perfused hearts, isolated heart tissue strips, quantitative morphometric analysis, gene expression, and biochemical markers of cardiomyopathy; (2) to determine if the DCM is influenced by the spatial or temporal nature of the Gi signal, by altering the anatomical location of the Gi signal, inducing continuous Gi signaling with a mutationally activated form of Gi, and reducing continuous basal Gi signaling by expressing a new RASSL (R2) that has a lower susceptibility to endogenous peptide agonists; and (3) to determine if a mouse heart with Gi-induced DCM can regain normal function on a physiologic, histologic, and cellular level.